There are four main crystalline forms of boron nitride: hexagonal, rhombohedral, cubic, and wurtzitc. Hexagonal boron nitride (hBN) and rhombohedral boron nitride are structurally similar to graphite with sheets of sp.sup.2 -bonded hexagonal arrays of boron and nitrogen atoms. Hexagonal boron nitride is an extremely soft, electrical insulator with a poor thermal conductivity. It is mainly used as a high temperature solid lubricant and refractory material. On the other hand, cubic boron nitride (cBN) and wurtzitic boron nitride (wBN) are structurally similar to diamond, with tetrahedrally coordinated frameworks of sp.sup.3 bonds. Consequently, cBN and wBN are extremely hard (the hardness of cBN is second to that of diamond), electrical insulators with excellent thermal conductivities ( the thermal conductivity of cBN is second to that of diamond). Cubic boron nitride is currently used as a powder for abrasive processes, and as sintered ceramics for sawing, cutting, and crushing applications. Sintered cBN is particularly useful as inserts in high speed machining of hardened steels, chilled cast iron, carbides, and nickel or cobalt-based super alloys. Unlike diamond, cBN has a low reactivity with iron and steels, and its high thermostability in oxidizing conditions makes it a better candidate than diamond for engineering materials. Cubic boron nitride has potential applications that include wear resistant and protective coatings, thermal heat sinks for electronic devices, electrical insulators in silicon based devices, lattice matched substrates for diamond growth. Unlike diamond which can only be doped p-type to date, cBN can be doped both n-type and p-type. Cubic boron nitride is therefore potentially more suitable than diamond in high temperature and high power electronic and optoelectronic devices.
Hexagonal boron nitride is most commonly grown either in film or powder form by chemical vapor deposition (CVD). Typically, reactants such as BCl.sub.3 and NH.sub.3 are combined on substrates heated above 1000.degree. in appropriate concentrations to form hexagonal boron nitride. The substrate temperature can be reduced considerably when the CVD process is excited by a plasma process such as hot filament, microwaves, or radio frequency waves. Cubic boron nitride films and powders are more difficult to produce. For example, cBN films are usually grown successfully only when a physical vapor deposition (PVD) process such as sputtering, e-beam evaporation, or laser deposition is modified with a beam of nitrogen or nitrogenic ions irradiating the target. These processes are either not easily scaled to high volume manufacturing (as in the case of laser deposition), or do not make films with adequate adhesion (sputtering and e-beam evaporation). Powders and crystals of cBN are produced by heating hBN to very high temperatures (2000.degree. K.) under considerable pressures (11 GPa). The formation of cBN in this direct conversion occurs via a destructive-reconstructive diffusion-like process. The temperature and pressure of the transformation can be reduced slightly by adding a catalyst such as an alkaline or alkaline earth metal. In this catalyzed process, compounds are added to hBN in order to reduce the high activation energy through a eutectic interaction with the BN. The driving force for the formation of cBN is the solubility difference between hBN and cBN varieties in the eutectic flux. The catalyzed process is the main method used to produce cBN at the industrial scale. Hydrogen has never been reported as an effective catalyst for the conversion of hBN to cBN.
Recently, researchers at Pennsylvania State University, under the leadership of Prof. R. Roy, announced that they had discovered a process that successfully changes graphite to diamond at ambient pressures. In their process, graphitic carbon is molded into different shapes and is seeded with diamond or nickel particles. The seeded parts were then placed into a hydrogen plasma and heated to temperatures between 600.degree. and 1000.degree. C. It is believed that the structure of the carbon changes in this process from graphitic to diamond. Roy speculated that the process may be simply a plasma enhanced CVD process where the hydrogen plasma etches the graphitic carbon, and redeposits it as diamond. The plasma enhanced CVD technique is known to work well in growing diamond coatings. However, a plasma enhanced CVD process does not work well in growing cBN, so the use of a hydrogen plasma would not appear to be beneficial to forming cBN. This invention goes against the teachings of Roy, et al.